I have read the beginning of this book and the little vikings must undertake a coming-of-age ritual. To do so, they must enter the cave of a live adult dragon and steal the eggs. So initially, the dragons are babies.
Therefore, the first poster is deceptive unless you realise that it grows and his task is to 'train' the untameable beast.
I think the movie will be quite charming in its own way and teach children that what they are afraid of often isn't really scary after all.